Although the present invention is applicable to many areas of technology requiring heat dissipating structures having low density and adjustable thermal expansion characteristics, its details will be described in terms of its application to electronics, and particularly with respect to the fabrication of heat-dissipating flip-chip covers or lids.
Nowadays, most electronic microcircuit components require the use of structures which are capable of dissipating the heat generated by the active parts of the microcircuit. Moreover, those structures in direct contact with one another must have compatible thermal expansion characteristics. Otherwise, stresses caused by the disproportionate expansion may damage components or reduce thermal dissipation efficiency.
The coefficient of thermal expansion ("CTE") or simply the thermal expansion of a material is defined as the ratio of the change in length per degree Celsius to the length at 25.degree. C. It is usually given as an average value over a range of temperatures.
The thermal conductivity ("K" or "TC") of a material is defined as the time rate of heat transfer through unit thickness, across unit area, for a unit difference in temperature or K=WL/AT where W=watts, L=thickness in meters, A=area in square meters, and T=temperature difference in .degree. K or .degree. C.
The constant drive toward further microcircuit miniaturization has resulted in the creation of the so-called "flip-chip" type of microcircuit as disclosed in Sugimoto et al., U.S. Pat. No. 4,698,663 and Nagesh et al., U.S. Pat. No. 5,585,671 incorporated herein by this reference. The flip-chip design offers lower electrical resistance, faster clock speeds, and lower cost assembly. Referring now to FIG. 1, a flip-chip integrated circuit die 1 has a plurality of solder bumps 2 arranged on its underside to electrically interconnect the die to a circuit board 3. Typically, an underfill layer 4 of electrically insulating epoxy further bonds the die to the circuitboard reducing mechanical stresses created by the die-to-circuitboard CTE mismatch. However, most commercially practical thermal epoxies have relatively poor thermal conductivity, typically ranging between 0.2 and 20 W/.degree.K.
Therefore, as shown in FIGS. 2 and 3, the need for greater heat dissipation resulted in the development of a heat dissipating cover or lid structure 5, also known as a heat spreader; which is bonded to the upper surface of the die 1 using a layer of thermally conductive adhesive 6 such as thermal epoxy. A stiffener 7 laterally surrounds the die and bonds the periphery 8 of lid to the circuit board thereby enclosing and protecting the die, preserving good contact at the die-to-lid interface, and allowing the use of non-adhesives such as thermal grease at the interface. FIGS. 3 and 4 shows that the stiffener may be integrated with the lid by forming the lid 9 to have a thickened periphery or flange 10 terminating at the inner edge walls 11,12 forming an underside cavity 13. FIG. 3 also shows the need for lids having a uniformly smooth and flat upper surface for intimately contacting a top mounted radiator 14.
Due to the relatively poor thermal conductivity of the interface layer between the die and the lid, minimum thickness is desired, leading to the need for a smooth, flat lid undersurface. However, if adhesive is used, a thinner epoxy layer is less capable of accommodating any expansion mismatch between the die and the lid, leading to breaks in the contact interface.
The use of flip-chip type integrated circuits with organic substrates having relatively low thermal conductivity makes thermally efficient lids even more critical.
In many applications such as aerospace electronics, overall reduction in weight is desirable.
As disclosed in Yamagata, et al., Development of Low Cost Sintered Al--SiC Composite, 1998 International Symposium on Microelectronics, Nov. 1-4, 1998, incorporated herein by this reference, it has been found useful to form such heat-dissipating structures from adjustable CTE, relatively high thermal conductivity, lightweight metal matrix composites such as aluminum-silicon-carbide ("Al--SiC"). Al--SiC is a metal matrix composite wherein silicon carbide ("Sic") particles are dispersed in an aluminum ("Al") or aluminum alloy matrix. The proportions of the Al to the Sic are selected to provide a compatible overall CTE while maintaining high thermal conductivity along with acceptable other characteristics such as homogeneity, smoothness and flatness, with good oxidative and hermetic stability.
Al--SiC composites having volume fractions of at least 20% Sic enjoy overall CTEs of less than 17.times.10.sup.-6 /.degree.C. which compares favorably with the CTE of the microcircuit or an intermediate buffer layer, while maintaining thermal conductivities in the range of about 130 W/m.degree. K to 210 W/m.degree. K and lightweight densities of between about 2.8 g/cm.sup.3 and about 3.0 g/cm.sup.3.
Another advantage of Al--SiC over other heat dissipating materials such as copper and most copper based metal matrix composites is that Al--SiC has greater oxidative stability, and does not require surface treatment as does Cu.
Although it may seem trivial, Al--SiC also has a smooth relatively light colored surface which readily carries ink printing. Copper based structures require an additional plating step to achieve such a surface.
However, current methods of manufacturing Al--SiC lids is expensive. One method disclosed in Yamagata, et al. Supra. involves thoroughly mixing aluminum and silicon carbide powders, compacting the mixture into a preform of the lid then sintering the preform.
Various other methods have been proposed for manufacturing heat-dissipating structures from AlSiC, such as disclosed in Yamada et al., U.S. Pat. No. 4,994,417 and Newkirk et al., U.S. Pat. No. 5,529,108, incorporated herein by this reference. These methods generally involve infiltrating molten aluminum or aluminum alloy into a porous preform of SiC particles. Other methods involve blendin Sic particles into moltan aluminum, then casting as disclosed in Hammond et al. U.S. Pat. No. 5,186,234.
Although infiltration, casting and mix-and-sinter methods can achieve near net-shape structures, those structures demanding high-tolerance dimensioning often require additional machining. Due the hardness and abrasiveness of SiC, Al--SiC composites containing even low volume fractions of SiC are difficult to machine. Even expensive diamond and carbide cutting tools exhibit rapid wear. Many heatsinks and heat-dissipating support or covering structures such as flip-chip lids have complex shapes and benefit from high volume fractions of SiC, making machining even more costly. Currently, a large amount of expensive machining is required to form the these finished structures even from preformed, near net shape Al--SiC structures.
Also, the composites themselves are subjected to stresses during machining which may cause chipping or other deformations. Thin structures such as flip-chip lids can have planar portions having surface areas of up to 2500 square cm and be between 1.1 and 4.0 mm thick, and are correspondingly more susceptible to machining damage.
Although the above techniques involve creation of the composite material in the near net-shape of the finished structure, this means that each lid is fabricated individually, from the start. Such individual processing can reduce manufacturing volume and increase overall cost.
Also, another drawback of infiltration methods is the typical formation of a "skin" of aluminum or aluminum alloy on the outer surface of the composite which can cause surface warping due to larger local CTE. Therefore, the skin is usually removed using an expensive machining step. This problem is worsened when near net-shape parts are made, since these parts may have more complex shapes and each part must be machined individually.
Therefore, the instant invention results from a need in the electronics field for high volume, low cost manufacturing of Al--SiC microelectronic heat-dissipating structures.